Metal oxide-hydrogen batteries, such as nickel oxide-hydrogen batteries, have seen wide use in aerospace applications because they are rechargeable, have an extremely long cycle life and provide a uniform output during the entire discharge cycle.
In the typical nickel oxide-hydrogen battery the positive electrodes are generally in the form of flat porous, sintered nickel plaques impregnated with nickel hydroxide, while the negative electrodes are formed of a fine nickel mesh screen having a catalyst, such as platinum black, bonded to one surface of the screen through a hydrophobic polymeric material. On discharge of the battery, hydrogen gas diffuses through the electrolyte surrounding the catalyst surfaces of the negative plates and becomes dissociated by the catalyst to the monatomic form. The monatomic hydrogen is ionized and combined with hydroxyl ions to form water with an electron being released in the process of forming each hydrogen ion. In addition, hydroxyl ions are formed at the positive electrode by the reaction of water with the available oxygen content of the nickel oxide. As a result of these reactions an electron current is produced in the exterior current.
On recharging the reaction is reversed, with the recharging being characterized by the regeneration of hydrogen gas at the negative electrode and the reoxidation of the nickel hydroxide at the positive electrode.
Due to the substantial gas pressures that are involved, the nickel oxide-hydrogen battery is contained within an outer pressure vessel. If the battery is used in a confined area or zone, such as in an aircraft or manned spacecraft, there is a potential danger in the event the hydrogen gas should leak from the outer vessel. Any leakage of hydrogen can result in explosive build up of hydrogen gas in the confined area when combined with oxygen in the air and an ignition source.